1. Field of the Invention
The present invention relates to a pickup actuator of an optical information recording/reproducing apparatus for recording and/or reproducing information on/from a disc by using optical means.
2. Description of the Related Art
The field of optical means for recording/reproducing information by using an optical disc, or the like, is expected to be the most demanded in the future. In such an optical recording/reproducing apparatus a pickup actuator is an important element since it seriously affects the information recording/reproducing speed. The pickup actuator includes a pickup base, a lens holder movably disposed with respect to the pickup base and has an objective lens mounted thereon, and lens holder driving means for moving the lens holder toward the directions for focusing and tracking.
An ordinary example of the pickup actuator is shown in FIG. 1. which will be described in greater detail below.
FIG. 1 is a perspective view of a conventional pickup actuator in which a reference numeral 10 refers to the pickup base. As shown in FIG. 1, the pickup base 10 has a body 12 and a holder support 14. The body 12 has a pair of yokes 20 and 20xe2x80x2 of a certain height disposed thereon while keeping a predetermined distance therebetween. Further, the body 12 is attached to the holder support 14 by a plurality of screws 16a and 16b. 
Further, a magnet 30 is attached to the inner side of the yoke 20. The magnet 30 forms a magnetic circuit, while the yokes 20 and 20xe2x80x2 function to concentrate and maximize the magnetic flux density of the magnet 30 toward the desired direction.
In FIG. 1, the reference numeral 40 refers to the lens holder. The lens holder 40 has the objective lens 42, and is movably suspended to the holder support 14 of the pickup base 10 by two pairs of wire springs 44a and 44b. Further, the lens holder 40 has a driving coil 50, which constitutes the lens holder driving means in cooperation with the magnet 30.
The driving coil 50 has a focusing coil 52 wound in a horizontal relation with the lens holder 40, and a pair of tracking coils 54a and 54b wound in a perpendicular relation with respect to the focusing coil 52. The focusing and tracking coils 52, 54a, and 54b cooperate with the magnet 30 to move the lens holder 40 in the directions for the focusing and tracking.
In such a pickup actuator, the objective lens 42 performs the focusing and tracking operations as the lens holder 40 is moved in the directions for the focusing and tracking by the mutual electromagnetic interaction of the magnet 30 and the focusing and tracking coils 52, 54a, and 54b. 
Here, as the objective lens 42 is moved in the directions for focusing and tracking, an undesired rolling in addition to a translational motion occurs, resulting in a degradation of optical signals and thus, causing the inaccurate reading of information or failure to record the information.
It has been known that the main cause of the rolling is due to inconsistency of following three centers, i.e., the center of gravity of the lens holder 40, the center of force of the electromagnetic circuit, and the center of support of the wiring springs 44a and 44b supporting the lens holder 40. In order to lower the possibility of rolling, these three centers are designed to be aligned with each other, to prevent the rolling when the lens holder is moved in the directions for focusing and tracking from its initial position, i.e., the neutral position.
However, when the lens holder 40 is moved in the focusing direction after having been moved in the tracking direction, or vice versa, the inconsistency of the centers of gravity and force may occur, resulting in the rolling.
The above will be described with reference to FIGS. 2 and 3.
FIG. 2 is a view showing the positions of the coils of the lens holder 40 of the conventional pickup actuator relative to the effective area 60 of the magnetic flux distribution when the lens holder 40 is at its initial position, i.e., at the neutral position, and FIG. 3 is a view showing the positions of the coils of the lens holder 40 of the conventional pickup actuator relative to the effective area 60 of the magnetic flux distribution when the lens holder 40 is moved in the focusing direction after having been moved in the tracking direction.
When the lens holder 40 is moved in the focusing direction after having been moved in the tracking direction, the portions of the tracking coils 54a and 54b which intersect the effective area of the magnetic flux distribution 60 in FIG. 2, cause unbalanced focusing directional forces of the lens holder 40 as shown in FIG. 3 (since the lens holder 40 is hidden by the focusing and tracking coils which are attached to the lens holder 40 in the figures, the lens holder 40 would be described referring to the coil area of the figures). As shown in FIG. 3, it is noticeable that a portion area X of the tracking coil 54a intersecting the effective area 60 is smaller than another portion area Y of the tracking coil 54b. This is due to a narrow width B between the tracking coils 54a and 54b. Accordingly, the unbalanced forces of the tracking coils 54a and 54b impedes to reduce the rolling of the pickup actuator.
Further, according to the conventional pickup actuator, since the magnet 30 and the yoke 20 are made to have widths C and A at a rate of 1:0.5-0.8, the width A of the yoke 20 is relatively narrower than the width C of the magnet 30. Accordingly, as shown in FIG. 4, the width of the uniform magnetic flux area is narrowed, again impeding the rolling control of the pickup actuator.
Meanwhile, to solve the above-mentioned problems, there have been several approaches such that the unbalance forces of the tracking coils in the focusing direction are balanced by laying a center line of the coils on a center line of the magnet, and such that the uniform magnetic flux area is expanded by shielding the leakage of magnetic flux using a cover covering the yoke, etc. The approaches, however, have shortcomings in that it can hardly balance the force just by coinciding the center lines of the coils and the magnet. Also, employing the cover to cover the yoke would result in increased number of parts, and thus, it inevitably increases the manufacturing cost.
The present invention has been developed to overcome the above-mentioned problems of the prior art, and accordingly, it is an object of the present invention to provide a pickup actuator having lower possibility of unbalanced forces of tracking coils in the focusing direction by minimizing the forces in the focusing direction of the tracking coil when the lens holder is moved in the focusing direction after having been moved in the tracking direction so that the rolling of the pickup actuator can be reduced.
Another object of the present invention is to provide a pickup actuator having wider uniform magnetic flux area of an electromagnetic circuit without using a separate part to decrease the rolling thereof.
Yet another object of the present invention is to provide a pickup actuator having a significantly reduced rolling of the area thereof by coinciding the center lines of the focusing coil and the magnetic flux of the magnet.
The above object is accomplished by a pickup actuator, including a pickup base, a lens holder having an objective lens mounted thereon, and lens holder driving means for driving the lens holder on the pickup base in the direction for focusing and tracking. The lens holder driving means includes at least one magnet, a pair of yokes attached to the magnet for concentrating and maximizing the magnetic flux density of the magnet toward the desired direction, and a driving coil having a focusing coil and a pair of tracking coils disposed between the pair of yokes.
Here, the magnet is attached to an inner side of one of the yokes, and the yokes have the same width as the width of the magnet. Accordingly, the uniform magnetic flux area of the magnetic flux can be widely distributed in the width""s direction of the electromagnetic circuit.
Further, the magnet is attached to one side of one yoke while being slightly lowered thereon, so that the center line of the magnetic flux of the magnet is aligned with the center line of the focusing coil. More specifically, the magnet is slightly lowered from its initial position to a distance corresponding to 4-12% of the height of the magnet on the yoke. Since the center line of the magnetic flux of the magnet is aligned with the center line of the focusing coils the possibility of unbalanced focusing directional forces by the tracking coils is reduced.
Further, in the pickup actuator according to the present invention, the width between the pair of tracking coils is wider than the conventional ones. Accordingly, the portions of the pair of tracking coils intersecting the effective magnetic flux area are positioned in the area where the density of the magnetic flux is low, so that the focusing directional force by the tracking coils can be reduced, and the possibility of unbalanced forces can be reduced.
In the pickup actuator according to one preferred embodiment of the present invention, the width of the magnet and the width of the yokes are expressed by the proportional expressions of C:A=1.0:0.8-1.2, where the width of the magnet is xe2x80x98Cxe2x80x99 and the width of the yokes is xe2x80x98Axe2x80x99. Here, it is most preferable that the width of the yoke is 1.0.
In the pickup actuator according to another preferred embodiment of the present invention, the gap between the pair of cracking coils and the width of the magnet are expressed by the proportional expression of B:C=0.6-0.9:1.0, where the gap between the pair of tracking coils is xe2x80x98Bxe2x80x99 and the width of the magnet is xe2x80x98Cxe2x80x99. Here, it is the most preferable that the gap B between the tracking coils is 0.75.
In the pickup actuator according to yet another preferred embodiment of the present invention, the magnet attached to one side of one yoke is lowered on the yoke, so that the center line of the magnet is downwardly moved to a distance E corresponding to 4-12% of the height D of the magnet on the yoke.
According to the present invention, the focusing directional force by the tracking coils is reduced, the uniform magnetic flux of the electromagnetic circuit is widely distributed in the width""s direction of the electromagnetic circuit, and the center line of the magnetic flux of the magnet and the center line of the focusing coil are exactly aligned with each other, so that the rolling of the pickup actuator can be significantly reduced.